1. Field of the Invention
The present invention relates generally to exchange route decision systems and methods of communication networks wherein tandem connection is carried out between various sorts of and multiple speed communication terminals through a plurality of exchanges to realize immediate information communication between the terminals and, more particularly, to an exchange route decision system and method which take residual line capacity into consideration.
2. Description of the Prior Art
FIG. 1 shows an example of a prior art exchange route decision method in the case where tandem connection is carried out between communication terminals through a plurality of exchanges. In the drawing, tandem connection between an outgoing terminal 10 and an incoming terminal 11 is realized by selecting either one of two routes, i.e., exchange 12.fwdarw.line 20.fwdarw.exchange 13.fwdarw.line 21.fwdarw.exchange 14.fwdarw.line 22.fwdarw.exchange 15.fwdarw.incoming terminal 11; exchange 12.fwdarw.line 20.fwdarw.exchange 13.fwdarw.line 23.fwdarw.exchange 16.fwdarw.line 24.fwdarw.exchange 15.fwdarw.incoming terminal 11.
The exchanges 12 to 16, which form a relay route for transmission of communication data, include connection type packet exchanges, connection type line exchanges, connection type asynchronous transfer mode exchanges and the like exchanges.
Prior to establishing the tandem connection between the outgoing and incoming terminals 10 and 11, the exchanges 12 to 16 determines one of the routes which is minimum in the cost C.sub.j of the routes in such a manner as to be mentioned in the following. Here, the cost C.sub.j is expressed in terms of an estimated value of the route from the outgoing exchange 12 to the incoming exchange 15, and defined, for example, as the following equation. ##EQU1## where Li represents the number of a line making up a route between the exchanges, D.sub.Li represents a distance between the exchanges, .alpha. represents the processing load cost of a relay exchange, and .beta. represents a cost coefficient relating to line distance. The value (.alpha.+.beta.D.sub.Li) indicates the load of the line and when the line is abnormal, the value is expressed in terms of .infin..
Each of the exchanges 12, 13, 14 and 16 calculates the costs C.sub.j of a plurality of routes from each of the exchanges to the incoming terminal 11. A minimum C.sub.Nj of the costs C.sub.j thus calculated as well as information on the number L.sub.x of starting one of the lines contributing to the minimum cost C.sub.Nj are held in a minimum cost table TBMC with respect to the different exchanges, as shown in FIG. 2. For example, assume that, in the example of FIG. 1, the lines 20, 21, 22, 23 and 24 have loads of 15, 15, 5, 25 and 10, respectively. Then the contents of the minimum cost tables TBMC for the respective exchanges 12 to 14 and 16 are as shown in FIG. 1. More specifically, when attention is directed to the exchange 13 which corresponds to a branch point of the two routes from the outgoing terminal 10 to the incoming terminal 11, the exchange 13 selects the line 21 having the line number Lx of L2 as a minimum cost line.
In the example of FIG. 1, accordingly, the route of the exchange 12.fwdarw.the exchange 13.fwdarw.the exchange 14.fwdarw.the exchange 15 is selected for the tandem connection between the outgoing and incoming terminals 10 and 11.
Such a route deciding procedure is shown in FIG. 3 in the form of a flowchart. More in detail, when the outgoing terminal issues a calling request, the present system retrieves the minimum cost line number Lx relating to the incoming exchange from the minimum cost table TBMC (step 30). Thereafter, the system examines whether or not the line capacity requested by the outgoing terminal remains in the line corresponding to the retrieved line number L.sub.x (step 31). If not, then the system processes it as a call loss.
When the line capacity remains in the line corresponding to the retrieved line number Lx, the system determines that the line of the retrieved line number L.sub.x is the minimum cost line (step 32) and calls the adjacent exchange which is connected to the downstream end of the L.sub.x line in question (step 33).
Here, assuming in the above route deciding procedure that the outgoing terminal 10 requests a line capacity q of 2 and the lines 20 to 24 have residual capacities Q (which can be used by the lines 20 to 24) of 4, 5, 1, 4 and 6, respectively, then the route extended from the exchange 12 to the exchange 14 can be used for tandem connection since the request capacity q of the outgoing terminal 10 is larger than any of the residual capacities Q of the lines 20 and 21. With respect to the route from the exchange 14 to the exchange 15, however, the residual capacity Q of the line 22 is smaller than the request capacity q and thus tandem connection is broken at the stage when the system calls the exchange 14, as shown by an arrow 25 in FIG. 1, whereby the system processes it as a call loss.
In this way, the prior art route decision method has had such a problem that, since the prior art selects one of the routes from the outgoing terminal to the incoming terminal on the basis of only the minimum cost information, there is a possibility that the prior art may select such a route as not satisfying the request line capacity of the outgoing terminal, thus involving a call loss.